How can I simulate metamerism in Cycles?
It's easy to demonstrate this effect in Blender. Cycles operates in RGB-only mode. It cannot simulate any colors in the visible light spectrum except for red, green, and blue.


This isn't correct.

The reference space primaries are determined by the colour management system. In theory, any reference space primaries within, or even beyond, the spectral locus can be selected. More specifically, any three light rendering engine can cover any area of the spectral locus, including imaginary coordinates beyond it, depending on the coordinates for each of the three lights.


  Is it possible to make colors in Blender act as their own true wavelengths/spectral distributions instead of a mixture of red, green, and blue? If you do that, layering translucent yellow and cyan planes will result in black, just like layering red and green.


This has nothing to do with spectral, and rather a difference between additive light and subtractive (multiplicative) light systems.


  Can blender render colors with custom/non-RGB spectral distributions?


If you mean "greater than three light" models, no. With that said, architecturally, a three light rendering engine isn't terribly far off from a spectral n-light rendering system; they crunch numbers in more or less the same way. The differences come in with respect to how the values are defined.

RGB however, isn't what it seems you think it is. RGB is literally any arbitrary set of three coloured lights, which are determined as a protocol layer in most instances in an raytracing engine. That is, the raytracing engine typically will just crunch numbers as though they were energy, and not care what colours are indicated by the intensities of the three lights. In this respect, RGB doesn't imply any specific colours of reddish, greenish, or blueish light, nor even that the labels represent reddish, greenish, or blueish lights but potentially something altogether of a different hue to what the somewhat arbitrary labels suggest.

There is nothing stopping someone from using spectral based primaries, such as those found in REC.2020, in Cycles for example. This is aside from the fact that Cycles and Eevee aren't properly colour managed, making the usage and interaction a bit tricky.